Comment by Animats | Hacker News Reader

Comment by Animats | Hacker News Reader

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This is cute, but the use case in robotics is questionable. A variable ratio transmission for a robot seems to solve a problem nobody has. Electric motor control today is able to do that job over a wide torque and speed range. From drones to electric cars to Diesel-electric locomotives, nobody uses variable-speed gearboxes much any more. There are many robot gearboxes, but I've never heard of one with a shifter. Machine Design has ads for robot gearboxes, including ads from Harmonic Drive, but they're all fixed ratio.

This thing would need a second motor to adjust the speed change, with an encoder, plus encoders on the input and output ends of the transmission. Something has to control all that.

If you need sudden bursts of high torque, overload the motor while monitoring temperature. You can run electric motors far above their continuous rating for brief periods, and if temperature is monitored, this is safe. That's how Tesla's "launch mode" works.

Not seeing the use case for this thing in robotics.

How do you feel about his claim that this would be good for energy efficiency?

Or the claim that this would allow for easier human-safety by having the torque, if lowered, not be dependent on logic, but on the actual mechanics, making it possible to stop the entire system if a failure in the torque-change motor is detected?

How do you feel about his claim that this would be good for energy efficiency?

It's unproven. If you have to haul more mass around, that's worse for energy efficiency.

easier human-safety by having the torque (limited)

There are mechanical 'fuses' for torque limiting.[1] Having a transmission that can downshift into a very high torque mode is probably worse for safety. If torque is directly proportional to motor current, there are lots of standard safety devices for limiting motor current.

[1] https://www.mayr.com/en/products/torque-limiters

Interesting, thanks for expanding. If you don't mind another question:

> a transmission that can downshift into a very high torque mode

I'm missing something here i guess, how is this different from a motor that can deliver full torque just because the logic chip is driving it at full power? Is it because the motor wouldn't be able to reach such high torque in the first place? (Which might be a point in favor of the transmission.)

Actually the gear shifting would also be done by logic, so nothing gained by that.

Not quite. In order to lower the torque on a stepper motor you need to have the logic controlling the motor control the torque for every single step the motor takes and that can switch from low to high between two steps. You can ward against this either with continuously checking logic or the torque limiters Animats mentioned: https://www.mayr.com/en/products/torque-limiters

If the inception drive is set to low torque a second motor needs to actively set it to a higher torque, which you can guard against by cutting power to the second one while in low torque mode, and having mechanical interlocks that cut power entirely if the shape of the drive changes to a high torque configuration while it's supposed to be low torque. Neither of these require continuous logic.

How cannot you have a continous checking logic in the driver logic of a gearless drive?

Let me help: you can, and I have taken part in developing one.

Link to patent?

Lol?

Electric servos for steering wheels use safety critical code, with lot of safety checks, and use direct drive. No need for silly patent for common sense out of the United States.

What's the difference between continous checking logic and safety critical code with lot of safety checks?

I'm not a mech engineer, i do code. I'm asking to learn and if you can prove me wrong by teaching something, yay.

Those safety checks need to be run continously, actually, or the software will not get the certifications necessary to be released on the roads. There are passive means: practices and coding guidelines, static checks, but also active measures: defensive coding, redundancy, continously active safety check logics.

So my original point was: it doesn't matter if you use direct drive or a transmission, as both will be controlled by software, and ultimately the safety of that software will determine whether the System is safe overall. The same design principles and safeguards will need to be implemented in both cases to provide the needed integrity.

But I doubt there are any patents on this. I guess it would be illogical to demand vendors to use patents by someone. But there are lots safety regulations on the topic.

Fair enough, and it makes sense. Thanks for explaining. :)

I think one of the very few remaining applications of variable-speed gearboxes are wind generators with no converter. But this in itself is rare, because it may be more reliable (?), but also less efficient than the power electronics... (afaik off-shore wind generators are going completely gearless).

Wind turbine gearboxes are all fixed ratio, as far as I can tell. The blade pitch is adjusted to maintain a constant speed in synchronous machines. Ones with AC-DC-AC converters don't need to do that. There's also something called a "doubly fed machine", which is a 3-phase generator where the field windings are actively driven to rotate the magnetic field to compensate for minor speed variations of the rotor.

Wind turbine gearboxes take a lot of pounding as the wind shifts. Wear is a big problem and not fully understood. The trend is toward gearless systems. Transmission repair atop a wind turbine is not fun. Power semiconductors are good enough that it's easier to deal with this electrically.

IIRC Voith developed these... I'm in no way an expert about these, but I'd guess that going gearless and using a slow-running generator would have the advantage that you get a much longer and fatter shaft to put bearings on.

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